An explosion followed by a start of fire occurred in the boiler room of a particle board manufacturing company subject to authorisation. Prior to the explosion, the press coupled with the boiler has already stopped and been restarted on several occasions. Just before 5.00 pm, the boiler 2 control room operator noticed that the entire facility had stopped operating and a white smoke/vapour” was released at the cooling oil pump. He immediately warned the area manager by telephone. A few seconds later, there was an explosion followed by a start of fire in the oil drain tank sector connected to the boilers primary circuit. Boiler room 2 was evacuated. The automatic foam sprinkler system brought the fire under control. The firemen on site assisted by external rescue teams put out the flames 15 minutes later in the secondary furnaces and marked out a danger area of 300m around the building whose sidings were on the verge of collapsing. To avoid the fire fighting water from polluting, the operated isolated the rain water collection tank from the water body where it emptied. The technician in the boiler control room who was under a state of shock was hospitalised. The siding sheets were ripped out during the explosion, two electrical cabinets and facilities related to the drain tanks were damaged by the flames. Even though the main frame of the building was not damaged, the operator feared that the explosion might have weakened the end restraints of the sidings. The inspector of classified facilities recommended the prefect to issue the operator an official notice ordering the update of the site contingency plan. Ten days before the accident, a start of fire had already broken out on the press of the plant (n° ARIA 29729). According to the expert analysis, water in the primary circuit of the boiler degraded the physico-chemical characteristics of the cooling fluid. The sudden overflowing of the hot oil into the drain tanks triggered foaming upon contact with the water in the tanks resulting in an overpressure in one of the tanks and opening of its rupture disc. The cloud that was vaporised exploded on contact with a hot surface.